Cell Chemistry of Sodium–Oxygen Batteries with Various Nonaqueous Electrolytes

Development of the nonaqueous Na–O₂ battery with a high electrical energy efficiency requires the electrolyte stable against attack of highly oxidative species such as nucleophilic anion O₂^•–. A combined evaluation method was used to investigate the Na–O₂ cell chemistry with various solvents, including ethylene carbonate/propylene carbonate (EC/PC)-, <i>N</i>-methyl-<i>N</i>-propylpiperidinium bis­(trifluoromethansulfonyl) imide (PP13TFSI)-, and tetraethylene glycol dimethyl ether (TEGDME)-based electrolytes. It is found that the TEGDME-based electrolytes have the best stability with the predominant yield of NaO₂ upon discharge and the largest electrical energy efficiency (approaching 90%). Both EC/PC- and PP13TFSI-based electrolytes severely decompose during discharge, forming a large amount of side products. Analysis of the acid dissociation constant (p<i>K</i>_a) of these electrolyte solvents reveals that the TEGDME has the relatively large value of p<i>K</i>_a, which correlates with good stability of the electrolyte and high round-trip energy efficiency of the battery

Tracking Formation and Decomposition of Abacus-Ball-Shaped Lithium Peroxides in Li–O₂ Cells

Study of formation and decomposition of Li₂O₂ during operations of Li–O₂ cells is essential for understanding the reaction mechanism and finding solutions to improve the cell performance. Using vertically aligned carbon nanotubes (VACNTs) directly grown on stainless steel meshes as the cathodes in the Li–O₂ cells with dimethoxyethane (DME) electrolytes, nucleation, growth, and decomposition processes of the Li₂O₂ in the first cycle are clearly visualized. Through cycles with the controlled discharge and charge capacities, the abacus-ball-shaped Li₂O₂ and the rust-like carbonates simultaneously formed around the VACNTs are further identified. It is indicated that the increasing coverage of carbonates on the cathode surface suppresses the formation of Li₂O₂, which maintains the shape of abacus ball. When the VACNT surfaces are predominantly covered by the carbonates, the cells tend to terminate

Sodium Storage and Pseudocapacitive Charge in Textured Li₄Ti₅O₁₂ Thin Films

Phase transformation reactions including alloying or conversion ones have often been utilized recently to improve the capacity performance of Na-ion battery anodes. However, they tend to induce larger volume change and more sluggish Na-ion transport at multiphase solid interfaces than for Li-ion batteries, leading to inefficiency of mixed conductive networks and thus degradation of reversibility, polarization, or rate performance. In this work, we use a structurally stable Li₄Ti₅O₁₂ spinel thin film as insertion-type model material to investigate its intrinsic Na-ion transport kinetics and coupled pseudocapacitive charging. It is found that the latter effect is remarkably activated by the nanocrystalline microstructure full of defect-rich surface, which can simultaneously promote Na-ion and electron accessibility to the surface/subsurface. It is proposed that the extra pseudocapacitive charge storage is a potential solution to the high-capacity and high-rate insertion anodes without trade-off of serious phase transformation or structural collapse. Therefore, a highly reversible charge capacity of 225 mAh g^–1 (exceeding the theoretical value 175 mAh g^–1 based on insertion reaction) at 1C is achievable

Influence of Gold Nanoparticles Anchored to Carbon Nanotubes on Formation and Decomposition of Li₂O₂ in Nonaqueous Li–O₂ Batteries

Gold nanoparticles (AuNPs) anchored to vertically aligned carbon nanotubes (VACNTs) act as additional nucleation sites for the Li₂O₂ growth, leading to the decreased size while increased density of Li₂O₂ particles in process of discharge. Correspondingly, at the deep discharge to 2.0 V the batteries show increased specific capacity. Upon charge, the AuNPs exhibit promotion effect on the Li₂O₂ decomposition by improving the conduction property of the discharge-formed particles, rather than by imposing the conventional electrocatalytic effect on the oxygen evolution reaction. Moreover, the AuNPs show promotion effect on decomposition of carbonate species arising from the side reactions. These effects consequently lead to the reduced charge overpotentials and extended cycle operation of the batteries. The results here provide a new as well as clear picture on the role of incorporated AuNPs in the Li₂O₂ formation and decomposition, which would be helpful for better understanding and constructing of high-performance air cathodes

Positive Role of Surface Defects on Carbon Nanotube Cathodes in Overpotential and Capacity Retention of Rechargeable Lithium–Oxygen Batteries

Surface defects on carbon nanotube cathodes have been artificially introduced by bombardment with argon plasma. Their roles in the electrochemical performance of rechargeable Li–O₂ batteries have been investigated. In batteries with tetraethylene glycol dimethyl ether (TEGDME)- and <i>N</i>-methyl-<i>N</i>-propylpiperidinium bis­(trifluoromethansulfonyl)­imide (PP13TFSI)-based electrolytes, the defects increase the number of nucleation sites for the growth of Li₂O₂ particles and reduce the size of the formed particles. This leads to increased discharge capacity and reduced cycle overpotential. However, in the former batteries, the hydrophilic surfaces induced by the defects promote carbonate formation, which imposes a deteriorating effect on the cycle performance of the Li–O₂ batteries. In contrast, in the latter case, the defective cathodes promote Li₂O₂ formation without enhancing formation of carbonates on the cathode surfaces, resulting in extended cycle life. This is most probably attributable to the passivation effect on the functional groups of the cathode surfaces imposed by the ionic liquid. These results indicate that defects on carbon surfaces may have a positive effect on the cycle performance of Li–O₂ batteries if they are combined with a helpful electrolyte solvent such as PP13TFSI

Charge Carrier Accumulation in Lithium Fluoride Thin Films due to Li-Ion Absorption by Titania (100) Subsurface

The thermodynamically required redistribution of ions at given interfaces is being paid increased attention. The present investigation of the contact LiF/TiO₂ offers a highly worthwhile example, as the redistribution processes can be predicted and verified. It consists in Li ion transfer from LiF into the space charge zones of TiO₂. We not only can measure the resulting increase of lithium vacancy conductivity in LiF, we also observe a transition from n- to p-type conductivity in TiO₂ in consistency with the generalized space charge model

Sustainable Interfaces between Si Anodes and Garnet Electrolytes for Room-Temperature Solid-State Batteries

Solid-state batteries (SSBs) have seen a resurgence of research interests in recent years for their potential to offer high energy density and excellent safety far beyond current commercialized lithium-ion batteries. The compatibility of Si anodes and Ta-doped Li₇La₃Zr₂O₁₂ (Li_6.4La₃Zr_1.4Ta_0.6O₁₂, LLZTO) solid electrolytes and the stability of the Si anode have been investigated. It is found that Si layer anodes thinner than 180 nm can maintain good contact with the LLZTO plate electrolytes, leading the Li/LLZTO/Si cells to exhibit excellent cycling performance with a capacity retention over 85% after 100 cycles. As the Si layer thickness is increased to larger than 300 nm, the capacity retention of Li/LLZTO/Si cells becomes 77% after 100 cycles. When the thickness is close to 900 nm, the cells can cycle only for a limited number of times because of the destructive volume change at the interfaces. Because of the sustainable Si/LLZTO interfaces with the Si layer anodes with a thickness of 180 nm, full cells with the LiFePO₄ cathodes show discharge capacities of 120 mA h g^–1 for LiFePO₄ and 2200 mA h g^–1 for the Si anodes at room temperature. They cycle 100 times with a capacity retention of 72%. These results indicate that the combination between the Si anodes and the garnet electrolytes is a promising strategy for constructing high-performance SSBs

Formation of Nanosized Defective Lithium Peroxides through Si-Coated Carbon Nanotube Cathodes for High Energy Efficiency Li–O₂ Batteries

The formation and decomposition of lithium peroxides (Li₂O₂) during cycling is the key process for the reversible operation of lithium–oxygen batteries. The manipulation of such products from the large toroidal particles about hundreds of nanometers to the ones in the scale of tens of nanometers can improve the energy efficiency and the cycle life of the batteries. In this work, we carry out an in situ morphology tuning of Li₂O₂ by virtue of the surface properties of the n-type Si-modified aligned carbon nanotube (CNT) cathodes. With the introduction of an n-type Si coating layer on the CNT surface, the morphology of Li₂O₂ formed by discharge changes from large toroidal particles (∼300 nm) deposited on the pristine CNT cathodes to nanoparticles (10–20 nm) with poor crystallinity and plenty of lithium vacancies. Beneficial from such changes, the charge overpotential dramatically decreases to 0.55 V, with the charge plateau lying at 3.5 V even in the case of a high discharge capacity (3450 mA h g^–1) being delivered, resulting in the high electrical energy efficiency approaching 80%. Such an improvement is attributed to the fact that the introduction of the n-type Si coating layer changes the surface properties of CNTs and guides the formation of nanosized amorphous-like lithium peroxides with plenty of defects. These results demonstrate that the cathode surface properties play an important role in the formation of products formed during the cycle, providing inspiration to design superior cathodes for the Li–O₂ cells

Lithium Expulsion from the Solid-State Electrolyte Li_6.4La₃Zr_1.4Ta_0.6O₁₂ by Controlled Electron Injection in a SEM

The garnet ionic conductor is one of the promising candidate electrolytes for all-solid-state secondary lithium batteries, thanks to its high lithium ion conductivity and good thermal and chemical stability. However, its microstructure is difficult to approach because it is very sensitive to the inquisitive electron beam. In this study based on a scanning electron microscope (SEM), we found that the electron beam expulses the lithium out of Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO), and the expulsed zone expands to where a stationary beam could extend and penetrate. The expulsion of metallic lithium was confirmed by its oxidation reaction after nitrogen inflow into the SEM. This phenomenon may provide us an effective probe to peer into the conductive nature of this electrolyte. A frame-scan scheme is employed to measure the expulsion rate by controllable and more uniform incidence of electrons. Lithium accumulation processes are continuously recorded and classified into four modes by fitting its growth behaviors into a dynamic equation that is mainly related to the initial ion concentration and ion migration rate in the electrolyte. These results open a novel possibility of using the SEM probe to gain dynamic information on ion migration and lithium metal growth in solid materials

Monodispersed Carbon-Coated Cubic NiP₂ Nanoparticles Anchored on Carbon Nanotubes as Ultra-Long-Life Anodes for Reversible Lithium Storage

In search of new electrode materials for lithium-ion batteries, metal phosphides that exhibit desirable properties such as high theoretical capacity, moderate discharge plateau, and relatively low polarization recently have attracted a great deal of attention as anode materials. However, the large volume changes and thus resulting collapse of electrode structure during long-term cycling are still challenges for metal-phosphide-based anodes. Here we report an electrode design strategy to solve these problems. The key to this strategy is to confine the electroactive nanoparticles into flexible conductive hosts (like carbon materials) and meanwhile maintain a monodispersed nature of the electroactive particles within the hosts. Monodispersed carbon-coated cubic NiP₂ nanoparticles anchored on carbon nanotubes (NiP₂@C-CNTs) as a proof-of-concept were designed and synthesized. Excellent cyclability (more than 1000 cycles) and capacity retention (high capacities of 816 mAh g^–1 after 1200 cycles at 1300 mA g^–1 and 654.5 mAh g^–1 after 1500 cycles at 5000 mA g^–1) are characterized, which is among the best performance of the NiP₂ anodes and even most of the phosphide-based anodes reported so far. The impressive performance is attributed to the superior structure stability and the enhanced reaction kinetics incurred by our design. Furthermore, a full cell consisting of a NiP₂@C-CNTs anode and a LiFePO₄ cathode is investigated. It delivers an average discharge capacity of 827 mAh g^–1 based on the mass of the NiP₂ anode and exhibits a capacity retention of 80.7% over 200 cycles, with an average output of ∼2.32 V. As a proof-of-concept, these results demonstrate the effectiveness of our strategy on improving the electrode performance. We believe that this strategy for construction of high-performance anodes can be extended to other phase-transformation-type materials, which suffer a large volume change upon lithium insertion/extraction